In chronological order in the past century, sails have been made of woven textile materials. Base fibers for these textile materials were derived from natural polymers, i.e., cellulose, of which cotton and linen were preeminent. In general, the fibers in these textile yarns used for weaving sailcloth were of short length as it is typically found in natural polymers. However, significant advantage in sails was realized by longer length fibers and high quality sails were sold as being made of long length "Egyptian cotton" yarns.
With the advent of synthetic fibers, that is an extruded bundle of "continuous" filaments for yarns, the length of fibers in yarns became immaterial, as typically all yarns were a bundle of "mono" filament yarns of substantial "fiber" length. Chopped fiber yarns or "spun yarns" were not used in sailcloth. Hence, the meaning of monofilament yarns, continuous filament fibers and yarns became interchangeable for sail making purposes. However, besides the fiber length in yarns, a synthetic filament in a bundle of monofilaments possessed many advantages such as initial modulus, tenacity, flex life, elongation at break, elongation resistance, resistance to creep, decay resistance, e.g., ultraviolet and mildew, weight-to-strength ration, etc. etc. These characteristics are for the modern filament yarns superior to the best cotton fabrics.
Accordingly, with the advent of continuous length filament fibers such as polyester and nylon (a polyamide), sailcloth are made of bundle of filament materials called yarns. Today substantially entirely all sails in economically advanced countries are made of synthetic fiber materials.
As new polymers were developed and as these lent themselves to filament formation and possessed the desirable properties for yarn formation, these materials found increasing use in sail making. For example, Kevlar.TM. (a polyaramid fiber sold by DuPont Co.) and Tawron.TM. (a polyaramid fiber sold by Akzo Co.) were used in sailcloth first with indifferent success, but as the fiber properties were improved such use became increasingly prevalent.
As new and improved derivatives of the above materials such as Kevlar 29.TM. and Kevlar 49.TM. and PEN polyesters (i.e., polyethylene naphtalate polymer) and entirely new synthetic fibers were developed with properties suitable for sail making, these materials found use in sails albeit at a very high premium over conventional polyester fiber fabrics. Examples of such monofilament materials are: Vectran.TM. (a polyaramid type of fiber sold by Hoechst-Trevira Corporation), Spectra.TM., Dyneema.TM., Certran.TM. (a high modulus polyolefin fiber sold by Allied Corp., DMS Company and Hoechst-Trevira Corporation respectively) and PBO (polyphenylene benzo bisoxazole) sold as Zylon.TM. by Toyoba Company. A considerable effort has also been expended to develop carbon fibers for sail making use, e.g., carbon fibers coated with a polyester or a polyamide polymer.
In sail making, when evaluating the above and novel fibers, the following tests are used:
Initial modulus: a measure of the yarn's ability to resist stretch. It indicates how well the fiber will hold shape, and is measured in grams of load per unit of stretch for a given denier. The higher the number, the less the stretch. Also defined as the slope of the initial straight portion of the stress-strain curve. PA1 Tenacity: The yarn's initial breaking strength, expressed in grams of force per denier. This is a good measure of a fiber's ultimate strength. The higher the number, the more load it takes to break the fiber. PA1 Flex life: A measure of the fiber's ability to retain its strength after being folded back and forth. It is expressed as a percentage of the fabric's strength lost after 60 bend cycles. PA1 UV resistance: Expressed as the amount of time it takes for a yarn to lose 50 percent of its modulus; normally conducted with artificial UV exposure. PA1 Elongation to break: A measure of the fiber's ability to resist shock loads. It is measured as how much a fiber will stretch (as a percentage of its overall length) before it breaks.
However, despite the advances in synthetic polymer technology, the inherent shortcomings associated with woven technology are evident, i.e., 90 degree warp and fill orientation and the over and under shape of the warp fibers caused by weaving called "crimp." These inherent shortcomings cause considerable problems associated with sail shape distortion. Shape distortion is caused by the anisotropic properties of the material when the force is applied at less than 90 degrees to the fill and/or warp orientation. It should be noted that typically sailcloth was woven with the better properties in the fill direction as the warp yarns, because of the "crimp" in the yarns, did not have the same elongation characteristics as the fill yarns. To remedy the inferior warp direction properties, "warp inserted" fabrics were also produced.
Within about the last 25 years considerable effort has been devoted to address the bias distortion in sails arising from the conventionally woven fabrics. This effort has had a three-prong approach. First, sailcloth manufacturers sought to improve the sailcloth by resin and heat treatment and resin applications. Additionally, sailcloth manufacturers added laminated films, typically a polyester film to the fabric on one, both sides, or in between two fabric layers. As the second approach, the sail makers employed panel orientation to align the fill threads with the load path, e.g., in tri-radial sails to minimize the bias inherent in a triangular sail typically used on recreational sailboats. Finally, as a third approach, sail makers devised structural sails (also known as fiber oriented sails) for racing; these were real "breakthrough" sails.
For structural sails, the initial development was to place the structure in the form of fabric strips, bundled monofilament fibers, i.e., yarns or yarns in the form of tapes on the skin or membrane of the sail. These added structures followed the load path in the sail. The load or stress maps for a sail had been available to sail makers for a number of years. The whole structure was typically confined either on one side or the other side or both sides of the sail. A subsequent development confined the structure between two layers of a film.
Bias distortion as used in the sailing parlance is typically caused by a load (also force or stress) that is "off-the-thread line." That is, if the warp (or ends) and the fill (or weft) fibers are in a line with the major, predominant load, sails are said to have the stress "on-the-thread" line," i.e., be less bias distorted. Typically, a sailcloth is woven with the fill threads under tension and therefore these do not suffer from the "crimp" of the warp threads. These fill threads are not as much subject to elongation as the warp threads when the sail is under load. However, in a typical sail there are other loads or forces "off-the-thread" line. By adding a laminated film to the material, typically a polyester film or a poly vinyledene chloride film (e.g. sold under a trademarks Mylar or Tedlar, respectively, and produced by a DuPont Company), bias distortion was reduced because these films display substantially isotropic properties. Improved polyester films such as PEN, (which is a polyethylene naphthalate polymer, i.e., a type of polyester polymer), may also be used in a film form and is also available as a fiber. Composite films of more than one polymer may also be used such as disclosed in U.S. Pat. No. 5,221,569. As previously mentioned, the yarns may be substantially immobilized by hot calendaring, resin impregnation, resin coating, as well as the laminating with the above-mentioned films. Reduced anisotropic characteristics are thus obtained. Nevertheless, in sailcloth, bias distortions cannot be entirely minimized by the above described steps as dynamic loading of a sail is still not easily quantifiable in the various sections of the sail.
To overcome or reduce the bias distortion, sailcloth manufacturers also resorted to multi-ply sailcloth materials. These efforts have been made towards improving the warp characteristics by producing the so-called "warp insertion" materials and also by inserting composites in the X direction (the machine or warp direction and opposite to the cross-machine or Y direction during manufacture) the so-called X-Ply materials or diaxial material (hereafter X-Ply). The X-Ply materials are an open mesh in a form of a scrim or a scrim supplemented by parallel yarns. These scrim materials which have a fiber orientation at 90 degrees or less, at various angles to the warp, are typically placed across the fiber carrying the major intended load, and are covered with a polyester film in the sailcloth material. These multiple ply materials often carry, as the X-Ply material, expensive fibers such as Technora.TM. of Teijin Company or Vectran.TM.. These multiple, composite materials carry the major load in the warp direction and are not only expensive but also rely on "over" design in the warp direction to over compensate for the bias distortion. Despite these weight and cost penalties, the X-Ply materials provide only, at best, an inexact, gross approximation to a load path when these materials are incorporated in a sail, typically in a gore form such as for tri-radial sails.
To minimize the cost of material and improve thread alignment, computerized nesting programs for cutting gores are available, i.e., for optimizing panel cutting such as for tri-radial sails. Still considerable wastage is experienced when making sails. Additionally, once distorted beyond a yield point, the films used in the laminate tend to break down or retain an irreversible shape without any recovery. Moreover, crinkling of the film and fabric composites and/or exposing these to sun also causes these materials to shrink to a greater or lesser degree. Bias distortion in these "panel optimized sails" is still introduced by the forces or stress exerted by aerodynamic loading of the sails as transferred to the "off-the-thread-line", and at boundary load concentration points, i.e., point loads of the sail. These stress concentration points consist primarily of a clew, head and tack points of the sail. Further, stress concentration is found at reef points, i.e., reef clew and reef tack, hanks, slides, battens, etc. In other words, the attachment means for the sail to a mast, stay, boom, or brace are typical stress concentration locations. These attachment points are also known as boundary point stress locations.
The reason for having repeatable consistency, i. e, properties in the warp, fill and bias e.g. 45 degrees direction for producing sailcloth and sails is made obvious when a distortion of two to four percent in a camber of a sail will result in significant performance differences. While a sail maker can measure the cloth properties in the machine direction and cross direction, i.e., or warp and fill yarns and has some confidence in the bias measurements, by experience, the consistency of available sailcloth material leaves a lot to be desired and leaves a sail maker at the mercy of a sailcloth manufacturer.
For the above reasons, the production of fiber oriented sails or structural sails (with added fabrics or scrim materials supplementing the primary yarns) has come to be regarded as the best present-day solution to the bias problem. These observations have been especially noticeable with respect to the high-end sails used for Grand Prix racing, e.g. America's Cup racing. However, the addition of the materials such as scrims and X-Ply materials to the fiber-oriented sailcloth has complicated already an essentially batch sailcloth and sail making process. Often, during sailcloth manufacture, each of the laminating, yarn insertion, and scrim insertion steps is a separate operation causing each to be a separate batch operation step with high labor content and with great increase in the cost of the sailcloth.
Still further, with the increased availability of the esoteric yarns, e.g., of fibers such as PBO, the cloth costs increase dramatically as represented by the actual yarns carrying the loads in a woven sailcloth. In the woven material, the yarns which do not carry the load are said to "run off" the material and are not continuous from panel to panel, i.e., are not joined along the curves of the load path. The "off-the-thread" material in essence only partially participates in the load bearing but contributes to bias distortion. Consequently, a great percentage of the yarns away from the 90-degree orientation in a cloth are carrying a disproportionately higher price versus their ultimate load-bearing capability. However, the recently adopted gluing of seams, as opposed to sewing, has displayed better load transfer properties between panels or gores.
When producing fiber-oriented sails, the sails are sought to be made with yarn orientation in the sail in a manner such that the properties in each section of the sail are predictable and properly balanced. For "balance" considerations, the starting point is based on the available stress maps or load-path maps which give the principal stress and/or principal load paths and stresses about perpendicular to the principal stresses known as secondary stresses or secondary load paths.
The most sophisticated software systems currently used for sail design combine a finite element analysis to model stresses within the sail membrane, with numerical flow codes to predict pressure variations over the curved sail surfaces. The two subprograms must be closely integrated because any sail shape change will alter the pressure distribution, and vice versa. Mainsail and headsail also interact aerodynamically to add another dimension of complexity.
Using these tools, a skilled designer can, in principle, fine-tune the curves of a sail so that the entry angles will harmonize with flow at every point up and down the luff as well as define the vertical camber at any location. Camber deflection analysis is also available as a design tool.
Using the computerized stress modeling, the engineering of the sail can be optimized in terms of fiber density and orientation. Areas of maximum load or potential overload can be identified and subsequently reinforced. By the same token, lightly stressed zones can be pared down in the quest to save weight for Grand Prix racing sails.
As discussed above, in a sail, in different parts thereof, stress is experienced in a multitude of different directions. In a woven sail material, the balance consideration of properties requires that the optimum or least anisotropic properties are consistent from one batch of sail material to the other. A good sailcloth is said to be "flat," i.e., has been weaved with consistent tension in the warp and fill, producing no "bumps" or "bubbles." Further, the material properties are said to be of the same value, i.e., magnitude, for example for modulus, stretch or elongation, bias distortion, etc. Any change or deviation from batch to batch of the sailcloth material (or fiber oriented sail material) distorts the sail unpredictably and causes the sail to perform unpredictably. Accordingly, if each sail material batch has different properties, the sail design cannot be made consistent. As mentioned above, by experience, it has been found that the horizontal depth or curvature of a mainsail, i.e., horizontal camber by as little as two to four percent will cause a significant change in the performance of the sail. Likewise, the change in the vertical camber will have drastic consequences in performance. The loss of performance is magnified if the curvature or camber migrates to a location in the sail different from that for which it was intended, e.g., towards the leach of the sail. For these reasons, eliminating variability and having predictable properties in a batch of conventional or fiber oriented sail material have been desiderata of all sail makers.
In the production of fiber-oriented sails, the consistency in yarn properties, the consistency of the structure, and the final laminate is just as much of importance as with woven sailcloth materials. As the design of the fiber oriented structure in a sail is still bound up with considerable intuitive art, the predictability, while significantly improved over woven-material sails, nevertheless allows for great improvements in the component parts of the structure. Although development of structural, i.e., fiber-oriented sails in effect freed the sail maker from the sailcloth manufacturer, it placed a greater burden on the sail maker to produce consistent materials. Some of the alleged improvements such as "round" fibers versus flat fibers, twisted fibers versus untwisted fibers, mixed fibers, etc. etc. have been more or less of defensive posturing type rather than based on proven results. Nevertheless, the reduced costs in a structural sails designed with substantially all of the fibers of the filament yarn type carrying the load has a been notable advance.
However, the experience on race courses has shown that initial fiber oriented sails were insufficiently strong when only primary yarns followed the load paths for the principal or primary stress. If no other than primary yarns were present and if the substrate, i.e., skin membrane was week, i.e., a polyester film, the sail was distorted. In other words, distortions due to aerodynamic loading had to be prevented by introducing a complex secondary structure, i.e., a strong membrane or secondary structural members to prevent distortion.
Distortions in fiber oriented sails appeared mostly but not exclusively in the horizontal direction, i.e., across the sail. Adding more primary yarn structure, and a scrim or taffeta combination has been an answer, albeit, an imperfect answer. Addition of scrim requires a separate manufacturing step and today two principal structural sail manufacturers, Sobstad, Inc., selling sails under the trademark Genesis and North Sails, Inc. selling its structural sails under the trademark 3DL, insert a layer of reinforcement, e.g., a scrim as a separate step in the sail/sail material manufacturing process. Both processes are not amenable to inserting a scrim as a bottom layer in a sail material during manufacture. The third structural sail manufacturer Ulmer-Kolius known as UK Sailmakers selling Tape-Drive.TM. sails uses a cross-cut panel sail of conventionally woven material or an X-Ply improved material to place a structure on it.